1. Field of the Invention
The present invention relates to a grey tone display and a driving method for the same and, more particularly to a display driving system utilizing complimentary thin film gate insulated field effect transistors suitable for used in liquid crystal displays.
2. Description of the Prior Art
There have been well known in the art active liquid crystal displays which are driven by thin film transistors (TFTs). The displays of this type comprise visual panels and peripheral circuits for driving the panel. The peripheral circuit is formed by attaching a single crystalline chip containing integrated circuits on a glass substrate by tab-bonding or COG (chip on glass). The visual panel comprises a plurality of pixels each being provided with a driving TFT. The TFT is usually an N-channel FET formed within an amorphous or polycrystalline semiconductor film which is electrically coupled to a respective pixel.
FIG. 1 is a diagram illustrating the equivalent circuit of an exemplary liquid crystal display. The diagram shows only a 2×2 matrix for the sake of convenience in description whereas ordinary liquid crystal displays consist of more great numbers of pixels such as those in the form of a 640×480 matrix, a 1260×960 matrix and so on. The liquid crystal display includes a liquid crystal layer 42 disposed between a pair of glass substrates 11 and 11′ as shown in FIG. 2. Numeral 54 designates a polarizing plate. The inner surface of the glass substrate 11′ is coated with a ground electrode 53. The inner surface of the other substrate 11 is provided with a plurality of conductive pads each constituting one pixel of the display. Each conductive pad are formed together with an n-channel FET 51 whose source is electrically connected with the corresponding pad. The drains of the FETs on a similar row in the matrix is connected with a control line of the row to which control signals are supplied from a row driver 47. The gates of the n-channel FETs on a similar column are connected with a control line of the column to which control signals are supplied from a column driver 46.
In the operation of the display, the column driver 46 supplies control signals of a high level to selected columns to turn on the TFTs on the column. There are, however, undesirable cases in which the on-off action of the TFTs can not sufficiently carry out so that the output voltage of the TFT (i.e. the input to the pixel) reaches only short of a predetermined high voltage level (e.g. 5V), or the output voltage does not sufficiently fall to a predetermined low voltage (e.g. 0V). This is because of the asymmetrical characteristics of the TFTs. Namely, the charging action on the liquid crystal layer takes place in a dissimilar manner as the discharging action therefrom. Furthermore, since the liquid crystal is intrinsically insulating, the liquid crystal voltage (VLC) becomes floating when the TFT is turned off. The amount of electric charge accumulated on the liquid crystal which is equivalent to a capacitance, determines the VLC. The accumulated charge, however, will leak through a channel resistance RSD which may be formed by dust or ionized impurities or through the liquid crystal itself whose resistance RLC 44 may be occasionally decreased. For this reason, VLC sometimes becomes at an indeterminate intermediate voltage level. In the case of the panel having two hundred thousands to 5 million pixels, a high yield can not be expected in such a situation.
Also, in the conventional driving methods, the liquid crystal material to which control voltages are applied is subjected to an average electric field in one direction during operation. The electric field tends to cause electrolysis when continuously used. Because of this, the aging of the liquid crystal material is accelerated and the life time of the display is decreased.
Furthermore, it has been difficult to realize grey tone display, without complicated structure, capable of arbitrarily displaying a variety of visual images in various shade of grey.